In a known vehicle braking technology, a stepping force of a brake pedal by a driver is detected and converted into an electric signal, and a brake fluid pressure is generated by a slave cylinder in response to the electric signal.
For example, Japanese Unexamined Patent Application Publication No. 2009-227023 discloses a brake device which includes a master cylinder connected to a brake pedal, a slave cylinder which operates depending on a stepping force of the brake pedal, and a Vehicle Stability Assist (VSA) (registered trademark) device connected to the slave cylinder. Further, a shutoff valve (master cut valve) is provided in a piping tube that connects the master cylinder and the brake cylinder.
In the disclosure of Japanese Unexamined Patent Application Publication No. 2009-227023, for example, when the vehicle is in an unstable state, the control unit stabilizes the behavior of vehicle by actuating the slave cylinder and the VSA device. During actuation of the VSA device or an automatic system such as a follow-up control, a low speed follow-up control and a collision mitigation brake that automatically actuates the slave cylinder, the control unit keeps the shutoff valve in a closed state. Moreover, when an abnormality such as a failure of an electric motor of the slave cylinder occurs during actuation of the VSA device or the automatic system, the control unit opens the shutoff valve so that stepping of the brake pedal is directly reflected to braking of the vehicle.
When the shutoff valve is opened, a pressure chamber of the slave cylinder communicates with a reservoir (having a pressure substantially equal to the atmospheric pressure). As a result, a pressure in an area upstream of a piston of the slave cylinder decreases to the atmospheric pressure, and the piston may not be fully returned (may not be fully retracted).
Accordingly, in the disclosure of Japanese Unexamined Patent Application Publication No. 2009-227023, during an initial stepping of the brake pedal after the shutoff valve is opened, the brake fluid pressure generated in the master cylinder is used to return the piston of the slave cylinder which is not fully retracted. That is, during the initial stepping, a certain amount of the stepping of the brake pedal by the driver is used to return the piston of the slave cylinder, which results in a fluid loss (ineffective stroke). When the fluid loss is large, the driver may feel a free running feeling.